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(AP) -- Efforts to care for abandoned cats could mean more humans will be exposed to rabies, researchers at the federal Centers for Disease Control and Prevention found.

For the past 30 years, the main domestic animal linked to human exposure to rabies has been cats. In the past 10 years, the number of feral cat colonies nationally has exploded as animal groups fight to end the capturing and killing of strays.

Those two trends could be on a collision course, said Charles Rupprecht, director of research for the Global Alliance for Rabies Control and senior author on the paper in the journal Zoonoses and Public Health.

Dogs were the primary domestic carrier of rabies until the 1970s, when aggressive rounding up of strays and vaccination programs finally eradicated canine rabies. That hasn't happened with cats.

"We didn't think it was OK to have (stray) dogs, but we think it's OK to create artificial cat colonies where they're exposed to wildlife that can transmit rabies," Rupprecht said.

Approximately 300 rabid cats are reported each year in the United States, said Jesse Blanton, an epidemiologist with the CDC and one of the paper's authors. It's estimated that 16% of people in the United States who undergo rabies treatment were exposed to the deadly virus from cats. They must be treated with a series of shots that costs about $2,000.

Human deaths from rabies are rare in the United States - two or three per year - and there have been no deaths linked to cats in decades.

The issue is part of a dispute over how unwanted cats and their offspring should be dealt with. There are about 74 million owned cats in the United States, according to the American Veterinary Medical Association. Estimates on the number of feral cats vary from 60 million to 150 million.

Many cat groups want an end to euthanasia in shelters. Because most feral cats haven't been socialized to live with humans and aren't suitable for adoption, they see creating outdoor colonies where they're fed and cared for as a humane alternative.

By trapping, neutering, vaccinating and returning (TNVR) these feral cats to the wild, the animals remain healthy and disease-free, and the colonies eventually die out, said Becky Robinson, president and co-founder of Alley Cat Allies, a national advocacy group for TNVR programs, based in Bethesda, Md.

The study authors dispute that the vaccination programs are thorough enough to end the threat. Cats have lower rabies vaccination rates overall compared with dogs; 38 states require vaccination for dogs vs. 30 for cats.

To be protected against rabies, cats need to be vaccinated once as kittens, then a year later and then at about three-year intervals, said Jane Brunt, a veterinarian and past president of the American Association of Feline Practitioners.

Capturing and recapturing feral cats two or three times during their lives to adequately vaccinate them is difficult, Rupprecht said. The paper suggests that the ability of TNVR programs to achieve the level of coverage necessary to protect those who come into contact with the stray cats "is doubtful."

There are hundreds if not thousands of these TNVR cat colonies. Alley Cat Allies has people caring for colonies in all 50 states. "We have half a million supporters," Robinson said. "Last year, we tracked more than 500 events on National Feral Cat Day."

Not everyone agrees with that approach. The Florida Department of Public Health has declared that feral cats are not tenable on public health grounds because of the disease threat. The American Association of Wildlife Veterinarians supports bans on feral cat colonies because of the risks they pose to wildlife and to human health.

The CDC paper disagrees with the notion that TNVR programs cause feral cat colonies to slowly decrease in size as cats die of natural causes. It cites studies showing that many continue to increase over time, either because not all animals are neutered or because of the arrival of new cats drawn to the food.

George Fenwick, one of the paper's authors and president of the American Bird Conservancy in The Plains, Va., said the public used to think that "unlimited feral cats was a bad idea." Now, he said, cat colonies are viewed as acceptable and even good.

Fenwick believes that more feral cats will mean "the incidence of rabies exposure is going to increase at a fairly rapid rate."

His involvement in the study is causing fur to fly among supporters of the colonies. The American Bird Conservancy has worked to spread research showing that outdoor and feral cats are responsible for killing and eating many more birds and wildlife than had previously been realized.

Cat supporters reject the paper's credibility because Fenwick's group "has been conducting a war on feral and outdoor cats for years," Robinson said.

Rupprecht said the paper's authors realized their topic was going to be controversial. "We waited a long time to publish," he said. Eventually, "we realized nobody else is going to do this. All we wanted to do was bring this up for discussion. We're not anti-cat."

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